This invention relates to tone decoders, and more particularly, to digital tone decoders which are capable of detecting the presence of one out of several possible frequencies in close proximity.
Some prior art tone decoders utilize linear tuned circuits centered about each desired frequency. The center frequency accuracy of each linear tuned circuit was required to be .+-.0.5% so that circuit stability and the ease of making initial adjustments were critical design parameters. This type of apparatus required periodic calibration and costly components. Another type of prior art tone decoder employing phase locked loop techniques was also used in certain applications, but the attack time was too long for satisfactory use in a high speed telephone dialing environment.
Prior art tone decoders also require additional circuitry to provide the ability to reliably detect the presence of a valid signal. Some prior art devices utilize a simple threshold comparator. If any energy, signal or noise, exceeded the predetermined threshold of the comparator, an indication of a valid signal was given. This technique can produce a false indication when only high level noise energy is present. Some other prior art devices determine the presence of valid input signal by comparing the energy in a narrow bandwidth around the expected signal frequency with the energy present across the total bandwidth of the device. The theory of operation here is that ambient noise energy has its spectral energy distributed over the entire channel, while a valid input signal would have its spectral energy distributed within a narrow bandwidth. When a valid signal was present the energy in a narrow bandwidth dominated the ambient noise energy present over the entire channel. When only noise was being received the energy in the wide bandwidth dominated. The disadvantage of using this technique is that it requires an additional bandpass filter and a detector.